This invention relates to improvements in high pressure mechanical seal assemblies constructed and used to prevent the uncontrolled leakage of a liquid along a rotating shaft, as for example, leakage along the shaft of a rotary liquid pump.
The present invention was developed especially for use with nuclear reactor coolant pumps, boiler recirculating pumps, boiler feed pumps and pipeline pumps, and will meet the requirements for extreme and widely changing conditions of pressures and temperatures encountered in these uses, it being understood that the present invention can also be used in less demanding installations. As an example of the widely changing pressures and temperature, the normal operating pressure in a pressurized water reactor is about 2200 psig, and during start-up, the pressure can be as low as 20 to 30 psig. In such reactor, the water in a coolant loop can reach a temperature of about 600.degree. F., while the water entering the seal area may be as low as about 80.degree. F. In these pumps, the pump shaft may move axially and may also wobble or deflect radially. An additional rigorous operating condition for the seals in such application is the combination of high pressure and high surface speeds which result from large diameter parts. It is thus necessary to construct a mechanical seal assembly capable of performing under these operational conditions.
Mechanical seal assemblies usually comprise the combination of a rotatable seal ring connected to a rotatable shaft for rotation therewith, and a non-rotatable or stationary seal ring connected to the flange of a housing. Each seal ring has a radial seal face and the seal faces oppose one another. Whether or not the seal faces engage one another is debatable because there is usually a film of fluid therebetween providing lubrication and cooling for the relative rotation between the faces. In some mechanical seals leakage across the seal face is controlled. In many seal assemblies, one or more coil springs urge one of the rings toward the other, so that in reality, one or both of the seal rings are capable of limited axial movement, even though they are commonly referred to as "rotatable" or "stationary". Many conventional mechanical seals can be used as single stage seals or in a multiple stage seal assembly.
In a common type of mechanical seal, one of the seal rings is constructed of a relatively brittle, soft material, such as carbon, whereas the opposing ring is constructed of a harder material, such as titanium carbide, silicon carbide, and the like. In many of these seal assemblies, the carbon ring is "backed up" by a back-up ring constructed of a harder material, such as a stainless steel. The mating faces of the relatively brittle, soft seal ring and the back-up ring are lapped, so that the carbon ring becomes stuck or "married" to the back-up ring. Because of the difference in the modulus of elasticity between the two materials of the seal ring and the back-up ring, i.e., carbon with a modulus of about 2.times.10.sup.6 to about 4.times.10.sup.6 and 18-8 stainless steel with a modulus of about 30.times.10.sup.6, a compressive load on the mated rings will cause the carbon ring to shrink more diametrically than the back-up ring. The carbon ring, being married to the back-up ring, will shrink more at its seal or running face, so that this face becomes concave which seriously affects the sealing area of the distorted face, leading perhaps to failure of the seal. The compressive load is mainly due to excessive differentials of the pressures on the inside and outside surfaces of the married rings, which frequently exist in the before-enumerated pumps.